1. Field of the Invention
The present invention relates generally to heat exchangers and more particularly to design aspects of heat exchanger components.
2. Background of the Invention
Although heat exchangers were developed many decades ago, they continue to be extremely useful in many applications requiring heat transfer. While many improvements to the basic design of heat exchangers have been made over the course of the twentieth century, there still exist tradeoffs and design problems associated with the inclusion of heat exchangers within commercial processes.
In particular, one of the most problematic aspects associated with the use of heat exchangers is the tendency toward fouling. Fouling refers to the various deposits and coatings which form on the surfaces of heat exchangers as a result of process fluid flow and heat transfer. There are various types of fouling including corrosion, mineral deposits, polymerization, crystallization, coking, sedimentation and biological. In the case of corrosion, the surfaces of the heat exchanger can become corroded as a result of the interaction between the process fluids and the materials used in the construction of the heat exchanger. The situation is made even worse due to the fact that various fouling types can interact with each other to cause even more fouling. Fouling can and does result in additional resistance with respect to the heat transfer and thus decreased performance with respect to heat transfer. Fouling also causes an increased pressure drop in connection with the fluid flowing on the inside of the exchanger.
One type of heat exchanger which is commonly used in connection with commercial processes is the shell-and-tube exchanger. In this format, the device is designed such that one fluid flows on the inside of the tubes, while the other fluid is forced through the shell and over the outside of the tubes. Typically, baffles are placed to support the tubes and to force the fluid across the tube bundle in a serpentine fashion.
Fouling can be decreased through the use of higher fluid velocities. In fact, one study has shown that a reduction in fouling in excess of 50% can result from a doubling of fluid velocity. It is known that the use of higher fluid velocities can substantially decrease or even eliminate the fouling problem. Unfortunately, sufficiently high fluid velocities needed to substantially decrease fouling are generally unattainable on the shell side of conventional shell-and-tube heat exchangers because of excessive pressure drops which are created within the system because of the baffles. Also, when shell-side fluid flow is in a direction other than in the axial direction and especially when flow is at high velocity, flow-induced tube vibration can become a substantial problem in that various degrees of tube damage may result from the vibration.
Higher fluid velocities associated with tube-side flow may also be problematic. For example, in the traditional shell-and-tube arrangement, the higher fluid velocities associated with tube-side flow tend to cause erosion of the tube""s inner surface particularly at the tube inlet. At a fluid velocity of, for example, 8 feet per second, the inner surface of a brass tube may erode over the length beginning at the inlet and extending for 6 inches or more into the tube. As fluid velocities increase, the problem worsens both in terms of the length of tube subject to erosion and the speed at which erosion occurs.
Tube erosion could eventually undermine the integrity of the tube-to-tubesheet joints. At the extreme, erosion can cause perforation of the tube which ultimately results in mixing between fluids on the shell side and tube side of the exchanger.
Inner surface tube erosion is especially problematic in the shell-and-tube arrangement since once a significant amount of erosion takes place, it becomes necessary to replace or repair the tube. Since, in conventional shell-and-tube heat exchangers, the majority of the tube length subject to erosion is embedded within the interior of the tubesheet, repairs and replacement of the tubes are costly and time consuming. For example, it may be necessary to cut the tube adjacent to the interior surface of both tubesheets, extract the remaining pieces within the interior of the tubesheets, extract the middle portion of the tube (between the two tubesheets), and then clean the surfaces and install a new tube. As is known in the art, this is an arduous process which generally results in significant process downtime.
In addition to the tube erosion problem discussed above, existing shell-and-tube heat exchangers suffer from the fact that xe2x80x9cdead zonesxe2x80x9d and areas of fluid stagnation exist on the shell side of the exchanger. These dead zones and areas of stagnation generally lead to excessive fouling as well as reduced heat-transfer performance. One particular area of fluid stagnation which exists in conventional shell-and-tube heat exchangers is the area near the tubesheet proximate to the outlet nozzle for the shell side fluid to exit the heat exchanger. Because of known fluid dynamic behavior, there tends to exist a dead zone or stagnant region which is located in the region between the each tubesheet and each nozzle. This area of restricted fluid flow on the shell side can cause a significant fouling problem in the area of the tubesheet because of the nonexistent or very low fluid velocities in this region. As is known in the art the same problem as described above also exists within the region adjacent to the inlet nozzle.
According to a representative embodiment, the present invention comprises a novel heat exchanger configuration which preferably uses the axial flow direction for the shell-side fluid and in which dead zones and areas of stagnation are significantly minimized or eliminated and in which inlet region tube erosion is addressed by providing a sacrificial portion of tube length so as to make repair and replacement of the eroded portion of tubes significantly cheaper, easier and with minimal process interruption. Because axial flow is employed with respect to the shell-side fluid according to a preferred embodiment of the present invention, tube vibration problems are generally eliminated.
In one embodiment of the present invention, a novel heat exchanger is provided such that each of the plurality of tubes contained within the heat exchanger extends a predetermined distance beyond the exterior surface of the tubesheet. The extension of the tubes in this manner permits a length of the tubes located near the inlet portion of the tubes to be employed as a sacrificial section which may be easily replaced prior to the point in time at which inner surface erosion reaches a problematic level. Further, in the event tube erosion does occur in the sacrificial section according to the teachings of the present invention, it is not as significant a cause for concern from the operational standpoint.
In still another embodiment of the present invention, a cone section which connects the shell to the tubesheet assembly is provided in order to allow shell side fluid traveling towards the tubesheet to uniformly and circumferentially exit the tube bundle while minimizing low-flow zones.
In yet another embodiment of the present invention, the novel heat exchanger is formed to include a shell extension which is located such that the shell in the heat exchanger of the present invention extends beyond where the heat exchanger cone meets the shell and further towards the shell-side face of the tubesheet located near the shell side fluid outlet. This shell extension serves to force shell side fluid flow toward the tubesheet in order to further minimize dead zones and regions of low or non-existent fluid flow at or around the center-facing surface of the tubesheet in the region located near the shell side fluid outlet and shell side fluid inlet. The shell extension also limits and/or eliminates shell-side erosion problems because it provides a 360-degree entry and exit path for shell-side fluid flow instead of a configuration where shell-side fluid flows directly against the tube bundle.
In another embodiment, the heat exchanger tubesheet is formed such that a conical extension which is preferably centered at the center of the shell-side face of the tubesheet is present. This conical section serves to further reduce and/or eliminate a small region of stagnation which would otherwise be present in the heat exchanger of the present invention as a result of directional flow caused by the aforementioned cone section and shell extension of the present invention.
In yet another aspect of the present invention, standard size xe2x80x9coff-the-shelfxe2x80x9d heat exchanger modules are employed to maximize the benefits of the fouling reducing aspects of the present invention and to allow for very significant reductions in design time when preparing to implement processes. According to this aspect of the present invention, several smaller standard size heat exchangers may be employed in parallel or in series or in both parallel and series to achieve the desired process characteristics including meeting the necessary heat-transfer requirements.
As will be recognized by one of skill in the art, and as will be explained in further detail below, the present invention provides many advantages including a significant reduction of dead zones and low-fluid-velocity regions which would otherwise lead to significant fouling problems.